These two scientists are collecting and testing samples of water from the river behind them. One of the properties of the water they are testing is acidity. They want to know how acidic the water is because water that is too acidic can harm the health of water organisms.

Acids are ionic compounds that produce positively charged hydrogen ions (H
+
) when dissolved in water. Acids taste sour and react with metals. Bases are ionic compounds that produce negatively charged hydroxide ions (OH
-
) when dissolved in water. Bases taste bitter and do not react with metals. Examples of acids are vinegar and battery acid. The acid in vinegar is weak enough to safely eat on a salad. The acid in a car battery is strong enough to eat through skin. Examples of bases include those in antacid tablets and drain cleaner. Bases in antacid tablets are weak enough to take for an upset stomach. Bases in drain cleaner are strong enough to cause serious burns.

Q:
What do you think causes these differences in the strength of acids and bases?

A:
The strength of an acid or a base depends on how much of it breaks down into ions when it dissolves in water.

Concentration of Ions

The strength of an acid depends on how many hydrogen ions it produces when it dissolves in water. A stronger acid produces more hydrogen ions than a weaker acid. For example, sulfuric acid (H
2
SO
4
), which is found in car batteries, is a strong acid because nearly all of it breaks down into ions when it dissolves in water. On the other hand, acetic acid (CH
3
CO
2
H), which is the acid in vinegar, is a weak acid because less than 1 percent of it breaks down into ions in water.

The strength of a base depends on how many hydroxide ions it produces when it dissolves in water. A stronger base produces more hydroxide ions than a weaker base. For example, sodium hydroxide (NaOH), a base in drain cleaner, is a strong base because all of it breaks down into ions when it dissolves in water. Calcium carbonate (CaCO
3
), a base in antacids, is a weak base because only a small percentage of it breaks down into ions in water.

The strength of acids and bases is measured on a scale called the pH scale, which is shown in the
Figure
below
. By definition,
pH
represents the
acidity
, or hydrogen ion (H
+
) concentration, of a solution. Pure water, which is neutral, has a pH of 7. With a higher the concentration of hydrogen ions, a solution is more acidic and has a lower pH. Acids have a pH less than 7, and the strongest acids have a pH close to zero. Bases have a pH greater than 7, and the strongest bases have a pH close to 14. It’s important to realize that the pH scale is based on powers of ten. For example, a solution with a pH of 8 is 10 times more basic than a solution with a pH of 7, and a solution with a pH of 9 is 100 times more basic than a solution with a pH of 7.

Q:
How much more acidic is a solution with a pH of 4 than a solution with a pH of 7?

A:
A solution with a pH of 4 is 1000 (10 x 10 x 10, or 10
3
) times more acidic than a solution with a pH of 7.

Q:
Which solution on the pH scale
above
is the weakest acid? Which solution is the strongest base?

A:
The weakest acid on the scale is milk, which has a pH value between 6.5 and 6.8. The strongest base on the scale is liquid drain cleaner, which has a pH of 14.

Why pH Matters

Acidity is an important factor for living things. For example, many plants grow best in soil that has a pH between 6 and 7. Fish may also need a pH between 6 and 7. Certain air pollutants form acids when dissolved in water droplets in the air. This results in acid fog and acid rain, which may have a pH of 4 or even lower. The pH chart
above
and the
Figure
below
reveal some of the adverse effects of acid fog and rain. Acid rain not only kills trees. It also lowers the pH of surface waters such as ponds and lakes. As a result, the water may become too acidic for fish and other water organisms to survive.

Acid fog and acid rain killed the trees in this forest

Even normal (clean) rain is somewhat acidic. That’s because carbon dioxide (CO
2
) in the air dissolves in raindrops, producing a weak acid called carbonic acid (H
2
CO
3
), which has a pH of about 5.5. When rainwater soaks into the ground, it can slowly dissolve rocks, particularly those containing calcium carbonate. This is how water forms underground caves.

Q:
How do you think acid rain might affect buildings and statues made of stone?

A:
Acid rain dissolves and damages stone buildings and statues. The
Figure
below
shows a statue that has been damaged by acid rain.

Summary

The strength of an acid or base is called acidity. It depends on how much of the substance breaks down into ions when it dissolves in water.

Acidity is measured by pH, which is the concentration of hydrogen ions in a solution.

Acidity is an important factor for living things because most can survive only within a relatively narrow range of acidity.

Vocabulary

acidity
: Concentration of hydrogen ions in a solution.

pH
: Measure of the acidity, or hydrogen ion (H
+
) concentration, of a substance.